The electrical conductivity of the mantle beneath Europe derived from C-responses from 3 to 720 hr

The C-response, which connects the magnetic vertical component and the horizontal gradient of the horizontal components of electromagnetic fluctuations, forms the basis for estimating the conductivity-depth profile of the Earth. This paper describes new estimates of the C-response obtained from observatory hourly mean values. The Z:Y-method is applied, which means that the vertical component is used locally whereas the horizontal components are used globally by expansion into terms of spherical harmonics. A special effort is made to obtain unbiased estimates of C. When applied to 90 months of hourly mean values from about 90 observatories, the method yields consistent results for European observatories in the entire period range from 3 to 720 hr, and for two different source mechanisms (S and D-st). A good description of the source structure for individual time segments is essential. This was achieved by a separate spherical harmonic analysis of the data for each month (for D-st) or each day (for S), and by estimating a large number (120) of expansion coefficients. The results are interpreted by means of 1-D conductivity models, which show that the upper mantle has remarkably little structure, with a monotonic decrease of resistivity from 100 Ohm m near z = 200 km to 0.7 Ohm m below z = 1000 km.